Since the GN4 version 2.3.2762.x, you can search keywords using a control that displays and allows selection of suggestions from the associated keyword set, e.g.

  <searchForm>

    <ssc:and>

      ...

      <ssc:suggest path="gn4:keywords/gn4:item" 

                         attributeFullName="archiveObject.keywords" 

                         minChars="3"/>

The suggest search control works in a similar fashion to that created by the Suggest template used when editing a keyword value.

Keywords are entered as a comma separated list.